Effects Of Zeolite Structure And Acidity On The Alkylation Of Benzene With Methanol

Alkylation of benzene with methanol(MeOH)is an important route to obtain toluene(T)and xylene(X).By the method not only MeOH can be converted into high-value aromatics,but also the application of petroleum benzene can be broadened.In this study,ZSM-5,Beta and MCM-22 family zeolites were used to systematically study the influence of zeolite structure and acidity on the alkylation of benzene with MeOH.The aim is to develop the catalysts with desired activity,stability and regeneration repeatability for benzene with MeOH alkylation.The main feature of this paper is that the content of ethylbenzene in C8 aromatic product(EB/Cs)was used as an important index for the evaluation of catalyst.This is because that only the Cs aromatics with low ethylbenzene content could be used as raw material in the isomerization process for para-xylene(PX)production.The main contents of this paper are as follows:Firstly,the effects of reaction temperature,pressure,feeding velocity and benzene/MeOH mole ratio on the performance of H-ZSM-5,H-Beta and H-MCM-22 zeolite in the alkylation of benzene with MeOH were investigated.Then the catalytic activity,reaction stability and distribution of alkylation product over the three zeolites were studied under the optimal reaction conditions.Results showed that,both H-ZSM-5 and H-MCM-22 zeolites show higher reaction activity and stability in the alkylation of benzene with MeOH.By contrast,H-Beta zeolite exhibits not only low activity but also rapid deactivation.The order of the amount of the by-product ethylbenzene is H-ZSM-5>H-Beta>H-MCM-22.Therefore,considering the activity,and the stability,and ethylbenzene amount H-ZSM-5 and H-MCM-22 zeolites are better catalysts for the alkylation of benzene with MeOH.The performance of H-MCM-22 zeolite is superior to that of H-ZSM-5 zeolite due to its lower EB/C8 ratio.In order to further study the alkylation of benzene with MeOH over MCM-22 family zeolites,MCM-36 zeolites were obtained by swelling,intercalating and pillaring of MCM-22(P)zeolite.Both MCM-49 and MCM-56 zeolites were hydrothermally synthesized in the presence of cyclohexylamine template.By exploiting the cooperative stucture-directing effects of the seed and the cyclohexylamine template,pure phase MCM-49 and MCM-56 zeolites were hydrothennally synthesized by adjusting the SiO2/Al2O3 ratio by using silica sol and sodium metaaluminate as main raw materials.MCM-22,MCM-36,MCM-49 and MCM-56 are all layered zeolites with MWW topological structure,but show different interlayer structure.Results show that the product distribution of the H-MCM-22 family zeolites are very similar to each other.The common advantage is that the content of ethylbenzene in Cs aromatics is very low.(EB/Cs was less than 1%)Considering the activity and the stability,H-MCM-22 and H-MCM-49 are the better zeolites among the MCM-22 family zeolites.However,the stabilities of H-MCM-22 and H-MCM-49 zeolites are not good after coke-burning regeneration,which could be a serious challenge to their practical application.Finally,the study was focused on the effect of H-ZSM-5 zeolite acidity on the alkylation of benzene with MeOH.The purpose is to reduce the content of ethylbenzene in its C8 aromatic product(EB/C8).Firstly,the influence of the acid content and strength of H-ZSM-5 zeolite on the alkylation of benzene with MeOH was studied using H-ZSM-5 zeolite samples with different SiO2/Al2O3 ratios,the samples obtained by steaming at high temperature,and the modified nano-H-ZSM-5 zeolite via alkali metal ion impregnation.Then,the roles of internal and external surface acid sites of H-ZSM-5 zeolite in the alkylation of benzene with MeOH were studied.Liquid phase chemical deposition(CLD)of silica with tetraethyl orthosilicate was used to silanize the external surface of the nano-H-ZSM-5 zeolite.A combination of the impregnation of sodium nitrate solution and the ion-exchange of the tetrapropylammonium bromide solution was used to prepare the internal surface modified nano-H-ZSM-5 zeolite catalyst.Results show that the internal surface of H-ZSM-5 is the main place where the ethylbenzene by-product is formed.A nano-zeolite catalyst In-ZSM-5-1.9 wt.%whose internal surface Bronsted acid sites was partially modified by sodium ion,showed the benzene conversion of more than 24%,which is only 4?5%lower than that of parent H-ZSM-5 zeolite.Interestingly,in a nearly 400 h reaction test,the catalyst kept its activity stable.The catalyst also maintained its performance after the three times of coke-burning regeneration.What is more,the average value of EB/C8 is about 1 5%,which is significantly lower than that of the H-ZSM-5 nano-zeolite parent(about 32%).These results indicate that the alkylation of benzene with MeOH on the external surface of nano-ZSM-5 zeolite has important study value.